why is clock showing me NaN?

var Stopwatch = React.createClass({
    getInitialState: function(){
        return{
            running:false,
            elapsedTimpe: 0,
            previousTime: 0,

        }
    },

    componentDidMount: function(){
      this.interval = setInterval(this.onTick, 100);
    },

    componentWillUmount: function(){
        clearInterval(this.interval);
    },

    onTick: function () {
        if(this.state.running){
            var now = Date.now();
            this.setState({
                previousTime: now,
                elapsedTime: this.state.elapsedTime + (now - this.state.previousTime)
            });
        }
        console.log('onThick')
    },

    onStart: function(){
        this.setState({
            running: true,
            previousTime: Date.now()
        });
    },

    onStop: function(){
        this.setState({running: false});
    },

    onReset: function(){

    },

    render: function () {
        var seconds = Math.floor(this.state.elapsedTime / 1000);
        return (
            <div className="stopwatch">
                <h2>Stopwatch</h2>
                <div className="stopwatch-time">{seconds}</div>
                {this.state.running ? <button onClick={this.onStop}>Stop</button> : <button onClick={this.onStart}>Start</button>}
                <button onClick={this.onReset}>Reset</button>
            </div>
        );
    }
})